Northstar Engines and System Technical Discussion Discussion, 00 N* still rough idle after changing plenum, cleaning TB in Cadillac Engine Technical Discussion; What we all thought was a cracked plenum on my 115,000 mile, 2000 STS turned out to some loose rubber ...

00 N* still rough idle after changing plenum, cleaning TB

What we all thought was a cracked plenum on my 115,000 mile, 2000 STS turned out to some loose rubber on the side of the plenum. After removing the plenum and intake manifold, I could not see any cracks anywhere. I spent all day cleaning the TB from both sides, installed new intake gaskets, new fuel injector o-rings, and installed my new plenum anyway. I also tried a blash of MAF cleaner and installed a new air filter. My plugs were replaced with new OEM plugs about 500 miles ago. After all of this, I still have my rough idle.... the engine sounds fine, but sitting in the car or looking at the engine with the hood open, you can see the engine "shudder" and "wiggle" about every second or so. This is extra annoying at stoplights, as it can be felt in every seat.This "shudder" at idle has been getting worse since about 80,000 miles. Fuel economy dropped from about 27 highway to about 25 highway around 100,000 miles. Still no codes... and Fuel Pressure REgulator works just fine.

Any ideas? Should I check the EGR valve even though I'm not getting any codes?

Also, the PCV sounds fine (it makes the clunking sound) but should it fit so loose? It easily slides out of its rubber socket/slot/holder on the rear valve cover. Seems like it should be tighter, but I don't know for sure.

Any other thoughts on what I should check? The only other thing I can think of is to have my fuel injectors cleaned and maybe change the fuel filter.

Re: 00 N* still rough idle after changing plenum, cleaning TB

I have both PCV hoses connected, FPR hose connected, brake booster hose connected. I'm really not sure what more I can do vacuum-wise. However I am still concerned about how easily the PCV valve itself can slip out of its rubber slot in the rear valve cover. It's a little more tight once everything is warmed up, but when the engine is cold the PCV valve and its attached hose slips right out of there with a very light tug.

Re: 00 N* still rough idle after changing plenum, cleaning TB

Try replacing the PCV grommet. Clean the EGR valve. Test the FPR again, but this time add vacuum to the FPR with a hand held vacuum pump or by sucking on a vacuum hose connected to it. I remember McCowden was having a problem and the FPR seemed to test fine until I suggested adding vacuum. That in addition to the fuel pressure from the other side showed it faulty.

Re: 00 N* still rough idle after changing plenum, cleaning TB

Well, a buddy and I looked over the PCV and PCV grommet and it seems to be tight enough. Brakleen sprayed in the area doesn't make a difference, so there doesn't seem to be a vacuum leak around there. We also tested the FPR and it too seems to be fine. I have not yet tackled the EGR. Still no codes but I still have the rough idle (a sort of shudder or vibration of the entire engine, visible and can be felt in all seats... every second or so). MPG has improved by about 15% since the TB cleaning and new intake gaskets. Could the vibration be due to fuel filter? Broken motor mount? Bad O2 sensors? Fuel injectors? I'm thinking about sending the injectors to be tested/cleaned. I'm still surprised I don't have any codes. The only codes I have seen thus far are voltage related from when I had the battery disconnected. And one code from when I put the trans into neutral while I had cruise on. Any other ideas in case the EGR cleaning doesn't help?

Re: 00 N* still rough idle after changing plenum, cleaning TB

I replaced the plugs about 500 miles ago. New plugs and a jug of Techron didn't improve the rough idle, but they did improve my acceleration and 0-60 time! Replacing the plenum and intake manifold gaskets improved my fuel economy by about 15%, but didn't improve the rough idle.

Once my EGR gasket is here, I'll clean it and see if that helps. Hopefully that's the ticket and hopefully I can simply clean it, new EGR isn't cheap. FPR was tested with a vacuum pump and seems to work fine. If it's cheap enough I might replace it and its gasket (a new FPR gasket came with my fuel injector gasket kit). I will also send my injectors in to be tested and cleaned.

So do you think I should replace my fuel filter? Is there any way to test the coil packs (this is a 2000 with coil-on-plug)? The coil packs are pretty expensive to replace on only a hunch. What are the symptoms of a clogged cat or muffler? My engine and exhaust sounds normal and it still has tons of performance and acceleration. It's just the rough idle that bothers me. Is there a chance that O2, IAT, or MAF sensors could be causing this problem? I'm not getting any codes, but maybe the sensors are giving slightly off readings? Sensors are pretty expensive to replace on a hunch, though.

Re: 00 N* still rough idle after changing plenum, cleaning TB

Hey does yours do it all of the time at idle? Mine does it intermittently, some days its smooth as silk and other days it rocks a little every second or so, though no change in the exhust note. I tried the fuel filter and new plugs and it didn't make a difference. I'm leaning towards the EGR valve and will check it this weekend. I doubt the cams would make the problem intermittent on mine. If it isn't the EGR valve, I may have to buck up for a coil pack.

Re: 00 N* still rough idle after changing plenum, cleaning TB

My idle is always "rough" although some days it doesn't rock/shudder quite as much, might have something to do with temperature or humidity or who knows what. The rough idle started around 80,000 miles. I now have 115,500.

I'm going to clean my EGR valve as soon as the gasket arrives, I'm sure it's due for a cleaning anyway. I'm also thinking about changing the fuel filter, it too might be dirty anyway. Coils, injectors, and other more expensive options have made me a little gunshy, as I don't want to be replacing the more expensive things on just a hunch.

Re: 00 N* still rough idle after changing plenum, cleaning TB

I lost some posts.

OK my bosses 00 DTS with 110K had a terrible idle. I replace the cams in it and it idles smooth as glass

I first installed the rear cams and the software patch. I needed to run the car out of the shop we got busy. I got back to it about a week later to do the other cams. The car ran about the same as it did when it came in, but the check engine light was flashing (misfires). Sure enough the front 4 cylinder all were misfiring. I replaced the front cams and the car ran so smooth you couldn't even tell it was running. I used 2003 VIN Y cams in it.

The car was misfiring since NEW, but all the cylinders misfired evenly. When performing Crank Variance learn Procedure it compensates for that. That is why it did not set a DTC

Re: 00 N* still rough idle after changing plenum, cleaning TB

I'm chasing down the same problem on a 2000 STS with 50k miles. I inspected the boots, did a voltage breakover & peak current test on the plugs while they were in place, and replaced the rear coil pack when I saw the ground current change with temperature. I checked all the hoses and inspected the plenum gasket. I checked the fuel pressure when the FPR was under vacuum and under atmospheric pressure. I tried a couple sea foam treatments. I listened to the injectors with a stethoscope and measured all the injectors' resistances. I checked the EGR which was clean.